The invention relates to a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation of a first type;
a mask table for holding a mask;
a substrate table for holding a substrate;
a projection system for imaging a portion of the mask, irradiated by the projection beam, onto a target portion of the substrate.
In particular, the invention relates to such a device in which the radiation of the first type comprises particulate radiation (e.g. electrons or ions), X-rays or extreme ultra-violet radiation (EUV).
An apparatus of this type can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at one time, such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (usually, M less than 1), the speed xcexd at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205, for example.
Until very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment and/or leveling measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn reduces the cost of ownership of the machine.
When radiation of the first type impinges on the substrate, energy from the radiation will generally be absorbed by the substrate, causing localized heating of the target area (die) which is being irradiated at that moment. In contrast, the substrate area outside the die, which is not being irradiated at that moment, will not undergo localized heating in this manner. Substrate heating is thus highly differential in nature, and can consequently cause significant differential stress in the substrate, with attendant mechanical deformation (expansion/contraction). This deformation can have a highly detrimental effect inter alia on the so-called overlay performance of the lithographic apparatus, which term refers to the accuracy with which a second patterned layer (new layer) can be juxtaposed upon a first patterned layer (old layer) already present on the substrate.
Although this problem is, in principle, present to some extent in all currently produced lithographic projection apparatus (in which the radiation of the first type is, for example, ultra-violet (UV) radiation with a wavelength of 365 or 248 nm), its magnitude is usually not so great as to cause substantial under-performance of the apparatus. However, a very different situation applies in the case of next-generation lithography systems, in which the radiation of the first type comprises, for example, electrons, ions, X-rays or EUV radiation (i.e. UV with a wavelength in the range 10-25 nm, e.g. 13.6 nm); in such apparatus, localized heating of the substrate can be quite intense.
It is an object of the invention to alleviate this problem. In particular, it is an object of the invention to provide a lithographic projection apparatus in which the effects of differential heating of a substrate during exposure are reduced.
These and other objects are achieved in an apparatus as specified in the opening paragraph, characterized in that the apparatus further comprises:
a secondary source for supplying radiation of a second type, which can be directed onto the substrate;
control means for patterning the radiation of the second type so that it impinges on the substrate according to a certain pattern.
By suitable embodiment of the control means, the sum of the fluxes of the radiations of the first and second type at substrate level will cause an elevation of the substrate temperature which is substantially constant across at least a given area of the substrate.
In essence, the invention ensures that those parts of the substrate which fall in the shadow of the radiation of the first type are illuminated (and consequently heated) by radiation of the second type, and vice versa; however, appropriate choice of the photosensitive material on the substrate, and of the type (e.g. wavelength) of the radiation of the second type, will ensure that the photosensitive material will only be exposed by the first-type radiation, and not by the second-type radiation. Proper adjustment of the second-type radiation dose at substrate level ensures that the substrate surface is heated to a substantially uniform temperature over at least that area which is to be covered with whole dies, thereby combating differential heating effects. Use of the word xe2x80x9csubstantiallyxe2x80x9d in this context does not require exact uniformity of the substrate""s surfacial temperature (although this is, of course, generally preferred); rather, the invention strives to produce at least some smoothing (and preferably a significant smoothing) of the differential heating effects which would occur in the absence of the invention.
The current invention can be envisaged at different levels, thus determining the size of the xe2x80x9cgiven areaxe2x80x9d referred to in the penultimate paragraph above. For example, at a basic level, when one die (target area) on the substrate is being exposed to first-type radiation, the (whole) substrate area outside that die can be concurrently exposed to second-type radiation; this will be referred to hereunder as a xe2x80x9ccoarse-level correctionxe2x80x9d. On a deeper level, the invention can be applied within a given die: the shadowed (masked) areas within that die are then irradiated with second-type radiation while the rest of the area within the die is exposed to first-type radiation; this will be referred to hereunder as a xe2x80x9cfine-level correctionxe2x80x9d. It is also possible to combine a coarse-level and fine-level correction.
In a first embodiment of the apparatus according to the invention, the radiation of the first type is selected from the group consisting of electrons, ions, X-rays and EUV radiation. Lithographic systems employing such post-optical radiation are presently undergoing development and preliminary testing by several companies, in reply to the semiconductor industry""s continuing drive toward smaller feature sizes, and the consequent demand for greater lithographic image resolution. Preliminary investigations have shown that the use of such radiation types can lead to very substantial substrate heating, with the attendant risk of producing significant differential stresses in the substrate; consequently, the present invention represents a particularly important contribution to these technologies. More information with regard to post-optical lithographic apparatus can be obtained, for example, from:
U.S. Pat. Nos. 5,079,112 and 5,260,151, in the case of SCALPEL lithography employing an electron source;
U.S. Pat. No. 5,532,496, relating to another electron-beam lithographic apparatus;
European Patent Application EP 0 779 528 and U.S. Pat. No. 5,153,898, in the case of EUV lithography.
Nevertheless, application of the invention can also be envisaged for other highly energetic radiation types, such as UV radiation with a wavelength of 193 nm, 157 nm or 126 nm, for example.
In a particular embodiment of the apparatus according to the invention, the radiation of the second type is selected from the group formed by microwaves, infrared radiation, visible light, and ultra-violet radiation. As already stated above, the resist employed on the substrate must be substantially insensitive to the radiation of the second type. For example, in the case of electron-beam lithography and ion-beam lithography, it is possible to use one of the resists currently available for use with DUV radiation (wavelength: 248 nm); in that case, the employed second-type radiation must be one to which such DUV resists are insensitive, e.g. infra-red radiation, or visible light with a wavelength longer than yellow. On the other hand, in the case of an EUV resist, a near-UV wavelength may prove suitable for use as second-type radiation. The skilled artisan will appreciate this point, and will be able to deduce a suitable combination of resist and second-type radiation on the basis of a given first-type radiation. Moreover, it will be obvious to the skilled drawing on that teaching of the present invention artisan that:
the use of a radiation type such as microwave radiation or infrared radiation as a second-type radiation will generally only be appropriate in performing a coarse-level correction (see above). This is because these radiation types do not generally lend themselves to fine patterning/focusing at the resolution of the features typically present in a die;
the successful use of the chosen second-type radiation in performing a fine-level correction (see above) will depend on the resolution of the features being imaged using the first-type radiation; if this resolution is too fine, then the (less energetic) second-type radiation will be difficult to pattern/focus at the required resolution. In this respect, performing a coarse-level correction is much easier, since, in that case, the second-type radiation need only be patterned/focused to a resolution of the order of millimeters (the width of a die) or centimeters (the width of a wafer) rather than tens of nanometers (the width of the individual IC features within a die).
In a particular embodiment of the inventive apparatus, the control means comprise a stencil plate. As already stated above, the second-type radiation should impinge on those parts of the substrate that are in the shadow of the first-type radiation, and vice versa. This can be realized in a relatively simple manner by locating a stencil plate between the secondary radiation source and the substrate. In the case of a coarse-level correction, such a stencil plate will comprise an portion which shields a shadow-area the size of a die (or, in the case of a step-and-scan device, the size of the scanning slit-image on the substrate), while allowing radiation access to the region around the shadow-area; in use, the plate will be positioned, at any given moment during the wafer exposure, such that the shadow-area coincides with a subject die upon which imaging is occurring (with first type radiation), whereby second-type radiation is allowed to impinge on (all) areas of the wafer except the said subject die (in the case of a step-and-scan device, the second-type radiation will be allowed to impinge on the substrate area outside the (moving) slit area). In the case of a fine-level correction, on the other hand, at least part of the stencil plate should contain a pattern which is substantially a negative of the pattern to be imaged onto each die (using first-type radiation); the term xe2x80x9cnegativexe2x80x9d here indicates that the stencil pattern is an inverse or complimentary image of the pattern to be imaged on the die.
An alternative embodiment of the apparatus according to the invention is characterized in that the control means comprise:
programmable memory means, for storing information regarding a pattern to be projected from the mask;
scanning means, for scanning a radiation beam from the secondary light source over the surface of the substrate;
attenuator means, for adjusting the intensity of the secondary light source during the said scanning motion, on the basis of the information stored in the memory means, thus causing patterned irradiation of the substrate with radiation of the second type.
In this embodiment, a beam of second-type radiation is scanned over an appropriate area of the substrate and is concurrently intensity-modulated so as to produce a patterned dose. For example, during exposure of a given die with first-type radiation, the intensity of the scanning second-type beam is kept relatively high in the shadow-areas within the die, and relatively low in the other areas within the die. Outside the die, the intensity of the second-type radiation is kept relatively high; this can be achieved not only using the said scanning technique, but also using a stencil plate, for example.
In the apparatus discussed in the previous paragraph, the memory means can be provided in one go with a xe2x80x9cmapxe2x80x9d of the pattern to be projected from the mask; this can be done, for example, prior to an exposure session (batch), by loading a map-file into the memory means from a map library. Alternatively, the memory means can be programmed xe2x80x9con the flyxe2x80x9d, using a technique whereby, instead of providing a file with prior information regarding the mask pattern, such information is instead xe2x80x9cmeasuredxe2x80x9d and stored in memory. In this latter case, an embodiment of the apparatus according to the invention is characterized in that the control means comprise:
measurement means, for determining the patterned intensity distribution of radiation of the first type at a reference level between the radiation system and the substrate table;
patterning means, for patterning the output of the secondary source so as to produce a second-type patterned intensity distribution at substrate level which is substantially a negative of the first-type patterned intensity distribution determined by the measurement means.
Examples of such embodiments are given in the next paragraph.
A particular embodiment of the inventive apparatus as described in the previous paragraph is characterized in that:
the radiation of the first type comprises charged particles;
the measurement means determine the patterned intensity distribution of the charged particles on the basis of a current measurement at the reference level.
In an alternative version:
the measurement means determine the patterned intensity distribution of the charged particles on the basis of a secondary electron signal measurement at the reference level.
The charged particles referred to here may be electrons or ions, for example (or, in principle, even other particles, such as protons or muons). The said reference level may, for example, be one of the following:
(a) mask level;
(b) the level of an angle-limiting aperture located in the radiation path between the mask table and the substrate table;
(c) substrate level.
More information on this point is given in the Embodiments below.
The application of the current invention will alleviate the problem of differential heating of a substrate during use of a lithographic apparatus. However, the substrate will still tend to get hot as a whole, and this can also be undesired. Fortunately, such xe2x80x9cglobalxe2x80x9d heating of the substrate is alleviated by substantial heat removal through the substrate table on which the substrate is located.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively. It will also be appreciated that the apparatus according to the invention may, if desired, contain more than one substrate table and/or more than one mask table (so-called twin-stage or multi-stage machines).